The present invention relates to a power supply for a three-phase plasma heating unit of the type which includes a three-phase current transformer and converter units connected to the secondaries, with three-phase current bridges equipped with chokes or reactors being provided in a direct current circuit. A three-phase plasma heating unit will be understood to mean a set of three plasma torches connected in a three-phase current manner.
A regulatable power supply must be used to provide regulated current to a plasma torch. If alternating current is supplied, periods without current must not occur during current reversal since this would cause the plasma arc to break off. To prevent cooling of the plasma arc during current reversal and thus permit optimum refiring, the prior art employs an auxiliary arc between two electrodes disposed in the plasma torch, however, such an arc requires its own direct current supply.
German Offenlegungsschrift No.(unexamined, published application) 2,501,076 discloses a device in which the superposed arcs of a plasma heater, i.e. the auxiliary arc and the main or power arc, are powered by but a single alternating current source which also furnishes the direct current for the auxiliary arc. The direct current for the auxiliary arc is obtained by way of a rectifier arrangement from part of the secondary winding of a transformer connected to the alternating current source. This prior art circuit, however, does not have the current limitation of the main arc necessary with respect to reliable plasma torch operation. Moreover, if current is obtained from a single transformer winding, reliable and economical work with a favorable power factor is possible only within an extremely limited operating range.
To create uninterrupted current and voltage curves in poly-phase alternating current regulating systems equipped with direct current chokes, German Pat. No. 2,943,324 provides for the superposition of currents and voltages on the secondary part of a transformer and thus on the load so as to produce a current flow and voltage duration of 180.degree..
According to an article entitled "Stromversorgung fur Drehstrom-Plasmabrenner" [Power Distribution for Three-Phase Plasma Torches] by Karl-Heinz Eichler, in the German periodical "Konstruktion--Elemente--Methoden (KEM)" [Construction--Elements--Methods], May, 1985, D-7022 Leinfelden (FRG), pages 114, 117, a converter connected to a tap on the secondary of a transformer is used for short arcs having a low requirement for voltage and a high requirement for current. The converter is initially driven until the maximum voltage of this tap is reached. As the voltage requirement of the plasma heater increases, a second converter is actuated. This second converter is connected, in parallel on the direct current side of a follower circuit, to a transformer tap having a higher voltage and lower current. Within each halfwave, the converter at the higher tap takes the place of the converter at the lower tap for the conduction of current until finally, at maximum actuation, the converter at the higher tap is the only one carrying current. If the voltage requirement increases even more, a third converter is actuated, etc. The converters are controlled by thyristors. However, this circuit has the great drawback that it requires a separate converter unit for each secondary winding, which, in addition to a large number of semiconductors, also requires a considerable control means.
Finally, East German Pat. No. 100,606 discloses a current source employing a three-phase current transformer in which the arc path (cathode to a workpiece serving as the anode) of a plasma heater operated with direct current is connected, via a non-controllable rectifier in three-phase bridge arrangement to a secondary power winding of the transformer. The arc current is regulated in steps by switching the windings of the transformer.